A laser trapping technique is well known as a technique for trapping cells or DNA in liquids. This is based upon the following principle: Light is refracted or reflected at the surface of fine particles having a refractive index different from that of the surrounding medium, as a result of which there is a change in the momentum of the photons. This change in momentum is transferred to the fine particles according to the law of conservation of momentum, thereby producing a change in the momentum of the fine particles. As a result, a force acts upon the fine particles in the direction of the focal point of a laser beam and the particles are trapped at the focal point of the laser beam. The trapped particles can be manipulated by moving the focal point of the laser beam. Thus, since a laser beam can be focused by the objective lens of a microscope and fine particles trapped in the field of view, this technique is well suited to the manipulation of cells or DNA under a microscope.
Because the object of interest can be manipulated without making contact with it, this technique is characterized by the fact that it is not affected by mechanical accuracy and vibration as in the case of a mechanical manipulator. Further, it is also possible to use a so-called microtool such as microspheres of polystyrene as the object trapped. Manipulation of cells indirectly by causing them to attach themselves to a laser-trapped microtool also is being carried out.
Furthermore, the specifications of, e.g., Japanese Patent Application Laid-Open Nos. 5-168265 and 6-78572 propose applying a laser trap as the driving source of a pump or motor. The gist of these proposals is to trap a microrotor at a prescribed position using laser trapping and simultaneously apply a rotating force to the rotor by the radiation pressure of the laser beam (a change in momentum ascribable to reflection and refraction of light). The rotational driving force can be controlled by the strength of the radiation pressure of the laser beam. With these techniques, there is no need for a bearing because the rotor is trapped by laser trapping. Thus these techniques are advantageous in that they eliminate the effects of bearing friction and wear that were major problems in microdrive mechanisms according to the prior art.
Further, various driving mechanisms that use laser light are illustrated in the specification of Japanese Patent Application Laid-Open No. 4-322183.
On the other hand, however, the micromotors according to the prior art have the following drawbacks:
1) In a case where a laser beam is used both as the trapping light and driving light, a certain degree of laser-beam intensity is required for trapping. As a consequence, the driving light also comes to possess this intensity and therefore it is difficult to rotate the rotor at low speed and to stop the rotor. Further, angle of rotation cannot be controlled freely because the rotor is rotated by radiation pressure.
2) In a case where the laser beam is not used both as the trapping light and driving light, it is required that the rotor be irradiated with the laser beam from a different direction. As a consequence, the structure for laser-beam irradiation is a complicated one. Alternatively, it is necessary to provide separate means for supporting the rotary shaft.
Accordingly, the present invention is to provide an optical micromotor having a rotor that can be controlled flexibly and freely through a simple structure, a micropump that uses this micromotor, and methods of manipulating these.